The present patent application relates to a method for asymmetrically coating a tape-shaped carrier body with molten silicon for further processing into solar cells upon employment of a device wherein a melt vat accepting the silicon melt is provided, exhibiting a channel in its bottom part for guiding the tape-shaped carrier body, said channel having a slit-like opening and extending in the direction toward the melt surface, the tape-shaped carrier body wound on a supply reel below the melt vat being guided in the opening by said channel, then being moistened by the melt and the coated carrier body than being rewound onto a storage drum disposed above the melt vat in the area of the guide channel, and the invention also relates to apparatus for the practice of said method.
Coatings of carrier bodies with molten silicon are known, for example, from U.S. Pat. No. 4,171,991 and from the Abstract No. 327 by C. Belouet from the Electrochemical Meeting, St. Louis, USA, May 11 through 16, 1980. Whereas, given the former method, a thick fabric comprised of graphite strands is drawn through a melt or is dipped into said melt and is completely moistened by it, given the latter reference a substrate consisting of a graphite plate is conducted through an opening situated in the floor of the melt crucible. The danger thereby exists that molten silicon will run out through the opening in the crucible floor and the coating will thereby be interrupted.
A method which can be executed given a device of the type initially cited can be derived from the German OS 30 10 557 Al. The possibility is provided by means of this device of manufacturing a planar silicon body with a high throughput (1 m.sup.2 /min). A mesh carrier body consisting of graphite which is resistant to molten silicon and can be well-moistened by silicon is thereby surface-coated with molten silicon and is co-incorporated into the silicon body when the silicon crystallizes. If no special measures are undertaken, the carrier body is generally situated in the center of the planar silicon, i.e., the carrier body is symmetrically coated with silicon. For the following reasons, however, it has proven advantageous to coat the carrier body asymmetrically, i.e., to carry out the coating such that the carrier body is only coated at one side:
1. The employment of the silicon material is reduced to a minimum. Even given a thickness of the silicon layer of only 150 or, respectively, 200 .mu.m, the carrier body, given a mesh thread thickness of 50 or, respectively, 100 .mu.m, is outside of the effective part of the solar cell, i.e., outside of the cell area in which charge carriers are generated by means of light absorption (penetration depth .ltorsim.100 .mu.m). Since the carrier body lies outside of the penetration depth of the light, no reduction of the efficiency due to "shadowing" occurs. PA1 2. The carrier body can be directly incorporated in the back electrode. PA1 3. The crystallization growth obstacles proceeding from the carrier body and negatively influencing the surface growth can be largely suppressed. Quasi-mono-crystalline surface growth can thereby be achieved, this having a favorable effect on the efficiency of the solar cell.
A method for drawing a polycrystalline silicon layer onto a carbonized ceramic substrate is known from the German OS 30 36 104 Al wherein an asymmetrical coating of the substrate is achieved in that the substrate is withdrawn from the silicon melt in an attitude deviating from the vertical, whereby the heating ensues due to the lower, non-coated layer which is still immersed. As a result of conducting the carrier body obliquely up (angle relative to the vertical approximately 10.degree.), an asymmetrical coating of the carrier body is achieved which, given a vertical guidance of the carrier body toward the top, as is the case given the method of the type initially cited, leads to a symmetrical coating. Oblique drawing of planar silicon, however, particularly when drawing with an after-heating zone which is indispensible given fast drawing and for a high crystal quality of the silicon, is very involved technically and problematical.